Type Ia supernovae with bimodal explosions are common - possible smoking gun for direct collisions of white dwarfs

We discover clear doubly peaked line profiles in 3 out of similar to 20 Type Ia supernovae (SNe Ia) with high-quality nebular-phase spectra. The profiles are consistently present in three separated Co/Fe emission features. The two peaks are respectively blueshifted and redshifted relative to the host galaxies and are separated by similar to 5000 km s(-1). The doubly peaked profiles directly reflect a bimodal velocity distribution of the radioactive Ni-56 in the ejecta that powers the emission of these SNe. Due to their random orientations, only a fraction of SNe with intrinsically bimodal velocity distributions will appear as doubly peaked spectra. Therefore, SNe with intrinsic bimodality are likely common, especially among the SNe in the lowluminosity part on the Phillips relation (Delta m(15)(B) greater than or similar to 1.3; similar to 40 per cent of all SNe Ia). Such bimodality is naturally expected from direct collisions of white dwarfs (WDs) due to the detonation of both WDs and is demonstrated in a 3D 0.64-0.64M(circle dot) WD collision simulation. In the future, with a large sample of nebular spectra and a comprehensive set of numerical simulations, the collision model can be unambiguously tested as the primary channel for SNe Ia, and the distribution of nebular line profiles will either be a smoking gun or rule it out.